Apparatus dew point
The theoretical saturation temperature a cooling coil's surface sits at, used with the coil's bypass factor, the fraction of air that passes through without contacting the surface, to predict leaving air condition. Leaving air is a mix of air treated to the apparatus dew point and untreated bypassed air, never the apparatus dew point alone.
Also called ADP, bypass factor
What is apparatus dew point on a cooling coil?
Apparatus dew point is the saturation temperature a cooling coil's surface sits at. Leaving air condition is a mix of air treated to that temperature and air that bypassed the coil untouched, a split set by the coil's bypass factor.
Apparatus dew point: how it works
A coil surface at the apparatus dew point pulls any air touching it toward that saturation condition, but a fin bank has gaps, and whatever flows through those gaps never meets the metal at all. Bypass factor states what share of the entering-to-apparatus-dew-point gap survives into the leaving stream, which is why the reading downstream sits between the two extremes rather than at either one.
Where you meet the term
- In coil selection, where two coils of equal tonnage can carry different bypass factors and therefore deliver different leaving air conditions.
- On a psychrometric chart, where the entering-to-apparatus-dew-point line and the leaving air point on it are constructed directly.
- In manufacturer performance data, either published directly as a bypass factor or derivable from stated entering and leaving conditions.
How to check it
- Read entering and leaving conditions from the same test data. Apparatus dew point and bypass factor are only meaningful together, at one stated entering condition and airflow. Mixing figures from different test conditions produces an inconsistent leaving air prediction. Technician
- Check airflow against the bypass factor's tested condition. Bypass factor rises with airflow, since faster-moving air spends less time near the coil surface. A bypass factor read at the wrong airflow understates or overstates the real leaving air condition. Technician
Apparatus dew point: what it is not
- Leaving air temperature is not the apparatus dew point. It is a mixed condition set by the coil's bypass factor, always warmer and more humid than the apparatus dew point itself.
- A lower bypass factor is not free. It comes from denser fins or more rows, which raises the coil's air-side pressure drop in return.
Apparatus dew point questions
Does every coil manufacturer publish bypass factor directly?
Not always by that name. Where it is not stated, it can be derived from published entering and leaving air conditions at a stated test airflow using the same chart construction.
Can apparatus dew point be below freezing?
For ordinary comfort cooling, no, but for an aggressive dehumidification application the calculated figure can approach or fall below freezing, raising frost risk on the coil surface.
Why does a wet coil calculation differ from a dry one?
A wet, actively dehumidifying coil condenses moisture onto its surface, which is the condition the apparatus dew point and bypass factor describe. A dry coil, heating or sensible-only cooling, has no equivalent moisture-driven mixing to calculate.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, and the bin and degree-day methods used for seasonal energy estimation.
- Manual S residential equipment selectionACCA description of Manual S as the procedure that selects specific equipment against a completed Manual J load, using the manufacturer expanded performance data at local design conditions rather than the nominal tonnage on the badge, and applying separate sizing limits to sensible capacity, latent capacity, and heating capacity.